It is known that in silver halide photographic elements, silver ions can be reduced to form metallic deposits of silver. When these deposits are unintended, such as when a camera containing the element leaks light thus exposing the element, or when physical pressure is applied to the element's emulsion layers by, for example, a component of a camera, then they are termed fog.
Fog can be formed locally or generally. Described above are two ways in which to form local fog. General fog, which occurs more or less uniformly across an entire element or emulsion layer, is typically formed in response to the ambient conditions in which the element is stored. For example, many photographic elements are susceptible to general fog formation and sensitivity loss when they are stored for an extended period of time in conditions of high temperature and humidity. General fog may also be formed by action of reducing agents contained in the photographic elements.
For more than forty years it has been known that certain palladium salts, when incorporated into a photographic emulsion, stabilize the emulsion and impart to it an increased resistance to fog formation and sensitivity loss. Palladium glycine complexes, in particular, have been known to control fog formation and sensitivity loss in photographic elements stored under tropical conditions. Accordingly, such palladium glycine complexes are utilized in many photographic silver halide materials currently commercialized.
Known palladium complexes are described in U.S. Pat. Nos. 2,598,079 and 4,892,808; Soviet Union Patent 1,656,491; European Patent Application 0 572 022; and German Patent 1,157,077. In U.S. Pat. No. 2,552,229, the sensitivity, gamma, and fog-inhibiting effects of a series of palladium complexes are explored.
Although many known and currently utilized palladium complexes are useful in solving the problem of fog formation and sensitivity loss as a result of prolonged storage in tropical conditions, use of these complexes provides the modern film builder with a whole range of additional unanticipated problems. As it turns out, when previously utilized palladium complexes are incorporated into an aqueous coating composition that is to be coated in a photographic element, the palladium complexes tend to interact with the gelatin of the coating composition. Such interaction is believed to be due to bonding of the palladium with amino and amide groups found in gelatin. The interaction forms reversible cross-links which leads to a rise in the viscosity of the coating composition.
The viscosity rise in aqueous coating compositions as a result of the interaction of palladium complexes with gelatin is acceptable to a limited degree. However, when excessive, it can lead to the formation of palladium gelatinate slugs; and the level of slug formation is directly related to the frequency of coating defects.
Filters are commonly used in an attempt to trap slugs. When numerous slugs are present, though, the pressure drop of the filter tends to rise as slugs accumulate on the filter. This requires more frequent filter changes. In extreme cases, filters can become blocked by slugs, thus causing stoppage of the coating.
When slugs accumulate on a filter, there is also the possibility that some will be forced through the filter because of the influence of the higher pressure drop and fragmentation of the slugs as they age. If slugs are coated, they can cause a local change in the thickness of the coated layer, which consequently affects the thicknesses of adjacent layers. This, in turn, can impact the quality of the image reproduced in the photographic element.
In an effort to minimize the impact of slug formation on the quality of photographic elements, film builders have employed such techniques as splitting the palladium complex among multiple layers; maintaining emulsion melts at higher temperatures or for longer times so as to disperse the slugs; diluting the coating composition to reduce the gelatin and palladium concentrations, which can adversely affect drying and thereby coating speed; and filtering through on-line filters which, as described, need to be changed frequently to remove the slugs. Another technique used is to coat the palladium complex at a level below that which is optimal for stabilization.
Although attempts have been made to reduce the detrimental impact of palladium complexes, such attempts have proved inadequate and have resulted in photographic elements that are overly susceptible to fogging and/or slug formation. Accordingly, there exists a desire in the art to obtain the advantageous stabilizing and antifogging effects known to be achieved by use of certain palladium complexes, without obtaining the detrimental viscosity increasing effects that are also known to result from use of the same complexes.